Currently, owing to the remarkable advancement of electric and electronic technologies, electric and electronic devices having various functions are being developed.
For example, there are continuously developed various electric devices such as light-emitting devices for emitting light using an electrical signal, memory devices for storing information therein, short/long-range wireless communication devices, sensor devices and the like. Also, portable terminals, digital equipment or information communication devices such as display devices, cellular phones, MP3 players, digital cameras or the like are diversely developed by employing the above electric devices.
Among them, for example, in case of the memory devices, a NAND flash memory used intensively for storage devices for cellular phones, MP3 players, digital cameras or the like is a non-volatile memory device which has addressed and solved a shortcoming of a DRAM device performing a volatile operation.
Particularly, an organic bistable device (OBD) is a memory device which is constructed such that conductive single-molecular, low-molecular and high-molecular bistable resistant organic layers are sandwiched between an upper metal electrode and a lower metal electrode. Such a memory device is known as being based on a new memory implementation concept in which the entire structure of the memory device is made up of one transistor and one resistor (1T1R).
Currently, reliability improvement of the devices or equipment, miniaturization, lightness, thinness of the film, enhancement of degree of integration and portability, and minimal reduction of the manufacturing cost are taken most preferentially into consideration to develop the above electric devices and conduct researches to apply them to electronic equipment with a variety of functions.
Formation of an electric device on a flexible substrate is taken as an example of development of a technology to accomplish miniaturization, lightness and thinness of the film. By virtue of this technology development, the enhancement of portability and the reduction of the manufacturing cost can be achieved.
A use example of the flexible substrate includes a film speaker which is adapted to implement, in the form of a film, a speaker function in which an analog electric signal is converted into a sound wave is implemented using a characteristic in which a piezoelectric polymer film such as a polyvinylidene fluoride (PVDF) film generates a vibration in response to an electric signal.
The film speaker employs a piezoelectric film which generates mechanically vibration when an electrical signal is applied thereto. Also, the film speaker reproduces sound using an inverse piezoelectric effect in a non-magnetically driven scheme in which a magnet is absent and provides an advantage of capable of significantly reducing the weight and thickness of the speaker and the manufacturing cost.
In addition, a technology for applying the film speaker to a flat panel video monitor is disclosed in U.S. Pat. No. 5,796,854 (1998 Aug. 18) entitled “a thin film speaker apparatus for use in a thin film video monitor device” wherein a display device and a vibration-generating device are actuated in independent spaces, respectively, with them separated from each other, but not being not actuated as an integrated composite device.
Currently, the demand for miniaturization, lightness, thinness of the film, reduction of the manufacturing cost, improvement of portability and the like of the electronic equipment is gradually increasing. Therefore, there is an urgent need for an integrated composite device.
As an example of the integrated composite device, an attempt has been made to develop a multi-functional integrated composite device in which various devices such as a display device, a memory device or the like together with a thin film speaker (vibration-generating device)/vibration-sensing device are implemented on a single substrate by using a piezoelectric polymer substrate having flexibility to thereby share a flexible substrate.
The use of a flexible piezoelectric polymer substrate which has been known so far has been limited to a single purpose for use in a vibration sensor or a sound-generating apparatus (vibration-generating apparatus) such as a speaker or the like.
The reason for this is that since the polymer substrate is susceptible to heat and may be easily damaged by an organic solvent such as acetone or the like used in a photoresist (PR) process to remove a photoresist layer, it is impossible to apply a typical etching process to the polymer substrate, thereby making it difficult to implement a micron-sized electrode pattern. Also, the pattern of an electric conductor for use in an existing sound-generating device is not a precise micro pattern of a micrometer level.
A pattern formation technique for implementation of electronic devices on a flexible thermoplastic polymer substrate has not been established so far, and a pattern formation method using the same print scheme as illustrated in existing patents has a critical shortcoming in that the surface adhesion strength between an electrode and a substrate is weakened.
Particularly, a nano-imprint method illustrated in existing patents has a limitation in mass-production due to a lack of reliability and repeatability of the process as well as a degradation of adhesion strength at the interface between the electrode and the substrate. A laser induced thermal imaging (LITI) as a laser transfer method also has a limitation in providing a stable micron-sized electrode pattern on a flexible substrate surface.
Like this, there are various technical limitations in performing an effective patterning process for allowing a polymer substrate made of PVDF or the like to act as a substrate for an electronic device. As a result, the patterning technique is limited to a technical level in which an illuminant or a phosphorescent substance is merely coated on the PVDF substrate and a substantial implementation of the micron-sized electrode pattern is not suggested clearly.
As described above, a lack of a patterning process which takes thermal instability and low heat resistance of a thermoplastic polymer into consideration acts as a great barrier in fabrication of a multifunctional composite device. There is an urgent need for simultaneously achieving the maintenance of adhesion strength at the interface between the electrode and the substrate as well as the low-temperature pattern formation which are most fundamental and critical factors in the development of a flexible electronic device.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms the prior art that is already known to a person skilled in that art.